This invention relates to hollow article internal pressure forming apparatus and method of the type usable for finally outwardly forming or, so called, "bulging" hollow articles such as cup-shaped knobshells and the like. According to the basic concepts of the present invention, the apparatus and methods thereof are arranged such that the forces against fluid internally of the article being so formed tending to outwardly form or bulge the same and therefore also tending to separate the dies containing the article during such bulging are produced directly by forces, the reactive forces of which tend to maintain the dies together. Thus, by automatically maintaining the reactive forces always larger than the article internal fluid forces, there is never a tendency of the dies to separate during the bulging operation, thereby eliminating one of the major difficulties with the prior constructions and methods.
Many prior constructions and methods have heretofore been provided for the internal pressure forming of hollow articles such as cup-shaped articles and one of the major important uses thereof has been the manufacture of hollow knobshells, usually formed of brass or bonze, which are ultimately assembled into doorknobs as provided by the builders hardware industry. In the formation of knobshells, starting by blanking from flat sheet stock, a relatively extensive series of drawing and forming operations are used to ultimately produce both relatively complex configured and relatively simply configured knobshells which may be finally assembled into doorknobs. Furthermore, in almost all such methods, the final major forming operation is that known as a "bulging" operation.
Specifically, after the relatively extensive drawing and forming operations, the hollow knobshells are slightly out of round and otherwise not of the final desired precise configurations, frequently depending on the complexity thereof. For these reasons, directly preceeding the bulging opertion, the knobshells are deliberately formed very slightly undersize. In the bulging operation, internal fluid pressure is applied to the knobshells, usually by hydraulic fluid under pressure, and the knobshells are finally expanded outwardly against containing die surfaces so as to produce the knobshell final exact form.
In this bulging operation, it is obvious that split or separable dies must be used in order that the knobshells may be inserted into and contained by the dies during bulging, and ultimately removed therefrom. As a result, one of the major problems with the prior knobshell bulging apparatuses and methods has been just exactly how to properly maintain the containing dies together during the internal hydraulic fluid pressure application sufficient to complete the bulging operation. If even the slightest separation movement is permitted between the containing dies during such bulging, not only improper forming can result, but scratches and marks on the knobshell outer surfaces can result requiring later extensive surface polishing operations.
The prior successful knobshell bulging methods have, therefore, required the use of complex double-acting presses having opposed rams, one ram applying mechanical or hydraulic forces to maintain the split dies together and the other ram frequently actuating an hydraulic plunger for applying the internal hydraulic fluid bulging forces. Obviously such manufacturing press equipment is relatively expensive and the bulging operations comparatively costly.